1. Field of Invention
The present invention relates to a projection exposure apparatus which is employed in photolithographic processes to manufacture microdevices such as semiconductor elements, imaging elements, liquid crystal display elements and thin-film magnetic heads.
2. Description of Related Art
In the photolithographic process for the manufacture of semiconductor devices, a pattern on a mask is transferred on to a wafer or glass plate (hereinafter also referred to as a substrate) coated with a photo resist. This type of projection exposure apparatus, a step-and-repeat type exposure apparatus (hereinafter also referred to as a xe2x80x9cstepperxe2x80x9d), has been widely used in the past. This step-and-repeat type exposure apparatus reduces and projects the pattern of the mask all together on each shot region of the wafer for exposure. When it finishes exposing one shot region, it moves the wafer and exposes the next shot region. It successively repeats this operation. Further, a step-and-scan type exposure apparatus is also being developed which enlarges the exposure range of the mask pattern by limiting the exposure light from an illumination system to a slit shape (rectangular shape), using this slit light to reduce and project part of the mask pattern on to the wafer, and in that state making the projection optical system synchronously scan the mask and wafer. This step-and-scan type exposure apparatus has both the advantages of the transfer system of an aligned which transfers the pattern of the entire surface of a mask on to the entire surface of a wafer by equal magnification by a single scan and exposure operation and the advantages of the transfer system of the above stepper.
In such a conventional projection exposure apparatus, the optical elements constituting the projection optical system are held in a barrel which is fastened to a frame of the exposure apparatus. The conventional means for fastening the barrel had been to bolt a flange formed at its center to the frame. However, the two ends of the barrel were free and there was the problem that the two ends of the barrel would shake due to vibration transmitted from other apparatuses in the clean room where the projection exposure apparatus was installed, for example, fans for control of the temperature and humidity and other sources of vibration. This vibration transmitted from sources of vibration around the projection exposure apparatus was transmitted to the barrel and caused vibration there even if the frequency of the sources of vibration was small since there is a correlation between the natural vibration of the sources of vibration and the natural vibration of the barrel, so the problem of displacement of the optical axis was caused. In particular, when the projection optical system is held by a plurality of barrels like in a reflection and refraction projection system and the plurality of barrels are fastened to the frame, the relative vibration among the barrels has a large effect, so the intervals between the barrels and the relative positions in the direction of rotation change and remarkable deterioration of the imaging characteristics occurs.
Further, the resolution of an exposure apparatus is substantially proportional to the wavelength of the exposure light, so the wavelength of exposure is gradually being made shorter. The exposure light is changing from visible g-ray (wavelength 436 nm) and ultraviolet i-ray (wavelength 365 nm) by a mercury lamp to KrF excimer laser light (wavelength 248 nm). Recently, use of ArF excimer laser light (wavelength 193 nm) and F2 laser light (wavelength 157 nm) has been considered. Here, in the wavelength region lower than that of about ArF excimer laser light, that is, in the vacuum ultraviolet region below about 200 nm, absorption due to oxygen in the air occurs and ozone is produced or substances produced by a photo reaction with the organic substances or moisture in the air deposit on the surface of the optical elements, whereby the transmittance ends up falling. As a measure against this, for example, in an exposure apparatus using ArF excimer laser light, the gas inside the barrels of the projection optical system is replaced with helium (He) or another inert gas, i.e., and so-called purging is performed.
The helium used as the purging gas is highest in performance as an inert and safe gas, but is present in only extremely small amounts in the earth""s mantle and atmosphere and therefore is high in price. Further, it is low in atomic weight, so it easily leaks from joints in the barrels of the projection optical system and is consumed in large amounts, therefore increasing the operating cost of the exposure apparatus.
Further, if the wavelength of the illumination light becomes shorter, the types of glass materials able to withstand actual use due to the absorption of light become more limited. If the wavelength becomes less than 300 nm, the only glass materials able to be used in practice at the present time would be fused silica and fluorite. The Abbe numbers of the two are not sufficiently far apart for correction of chromatic aberration, so correction of chromatic aberration would become difficult. As opposed to this, a catadioptric system combining a reflection optical system using a concave mirror etc. and a refraction optical system has been proposed. This catadioptric system can substantially eliminate various types of aberration such as, first and foremost, chromatic aberration, without causing an increase in the number of lenses. In a catadioptric system, however, generally it is necessary to use an optical path deflection member to separate the outbound optical path going toward the concave mirror and the return optical path from the concave mirror. As a result, several optical axes end up present. For this reason, when assembling the catadioptric system, there is the problem that error easily occurs in the adjustment among the plurality of optical axes compared with a refraction optical system.
Further, if the optical path deflection member of the catadioptric projection system tilts (rotational motion), large rotation and distortion of the image occurs. To stably obtain an image of an extremely high resolution, tough requirements are placed on the assembly position of the optical path deflection member. Further, tough requirements are placed on prevention of vibration as well so that the optical path deflection member is held in that position.
Accordingly, a first object of the present invention is to provide a projection exposure apparatus able to reduce vibration of a barrel structure of a projection optical system and thereby realize good imaging characteristics.
A second object of the present invention is to provide a projection exposure apparatus able to reduce the amount of the inert gas etc. used when supplying said gas to an optical path of the illumination light without causing deterioration of the imaging characteristics.
A third object of the present invention is to provide a projection exposure apparatus able to realize easy adjustment of a plurality of optical axes.
A fourth object of the present invention is to provide a projection exposure apparatus resistant to the occurrence of rotation of the image even when an optical path deflection member tilts and therefore stably giving an extremely high resolution.
1. To achieve the first object, according to the present invention, there is provided a projection exposure apparatus for transferring a pattern of a mask on to a substrate, said projection exposure apparatus comprising a projection optical system having an optical element, a frame for supporting a barrel structure holding the projection optical system, and a support member for supporting the barrel structure at a position different from the frame. The barrel structure may also be a plurality of barrels holding the projection optical system. In the case of a plurality, each barrel may be supported by the frame and the support member. The plurality of barrels for example may include a first barrel for holding a first optical system, a second barrel for holding a second optical system having an optical axis substantially parallel to an optical axis of the first optical system, and a third barrel for holding a third optical system arranged between the first optical system and the second optical system and having an optical axis substantially perpendicular to the optical axis of the first optical system. The support member is not particularly limited, but for example may be integrally attached to the frame.
In the present invention, the barrel structure for holding the projection optical system is supported by the frame. Simultaneously, the barrel structure is supported by the support member at a position different from the frame. Therefore, the end of the barrel structure which originally would become free is fastened. Accordingly, the occurrence of vibration at the barrel structure is suppressed and displacement of the position of the optical axis or rotation of the optical system held by the barrel structure is prevented. In particular, when the barrel structure comprises a plurality of barrels holding the projection optical system, the relative positions among the optical axes of the optical systems held by the barrels are extremely important. By using a support member to support the barrels individually or integrally, however, the vibrations of the barrels are suppressed and the intervals among the optical axes of the optical systems held by the barrels and relative positions in the direction of rotation are restrained. The barrel structure is preferably fastened to the support member through a connecting member having flexability. The barrel structure will sometimes expand or contract due to changes in the temperature environment. In this case, if the barrel structure warps as a result of the restraint of the support member, there would sometimes be a detrimental effect on the optical characteristics, but by interposing a connecting member having flexability, it is possible to keep an unnecessary force from acting on the barrel structure and at the same time suppress vibration.
2. To achieve the first object, according to the present invention, there is provided a projection exposure apparatus for transferring a pattern of a mask on to a substrate, said projection exposure apparatus comprising a projection optical system having a first reflection optical element for bending an optical axis, a first optical system arranged between the mask and the first reflection optical element, and a second optical system arranged between the first reflection optical element and the substrate, a frame for integrally supporting a first barrel for holding said first optical system and a second barrel for holding the second optical system; and a connecting member connecting said first barrel and said second barrel at a position different from the frame. In the present invention, the first optical system and the second optical system comprising the projection optical system are held by the first barrel and the second barrel respectively, and the first barrel and the second barrel are integrally supported by the frame, but simultaneously the first barrel and the second barrel are connected by the connecting member at positions different from the frame, so the ends of the first barrel and the second barrel which would have become free are mutually fastened. Accordingly, the vibrations of the first barrel and the second barrel will no longer affect each other and the interval between the optical axis of the first optical system held by the first barrel and the optical axis of the second optical system held by the second barrel and the relative positions in the direction of rotation are restrained.
While not particularly limited to this, preferably said second optical system has a second reflection optical element for bending said optical axis, an optical axis between said second reflection optical element and said substrate becomes substantially parallel to the optical axis of the first optical system, and the connecting member connects the first barrel and part of the second barrel between the second reflection optical element and said substrate. If the optical axis between the second reflection optical element and the substrate (hereinafter also referred to as the xe2x80x9cimage plane side optical axisxe2x80x9d) and the optical axis of the first optical system (hereinafter also referred to as the xe2x80x9cobject plane side optical axisxe2x80x9d) are made substantially parallel, the first barrel and the part of the second barrel will become substantially parallel. Since the first barrel and the part of the second barrel are connected by the connecting member in this state, in other words since barrels for which parallelism is desired to be maintained are directly connected, it is possible to better prevent relative displacement of the optical axes between the object plane side optical axis and the image plane side optical axis. Further, if the first barrel and the part of the second barrel become substantially parallel, there is also the advantage that the connecting structure by the connecting member becomes simplified.
In this case, while not particularly limited to this, the second barrel may be comprised to include two barrel components for holding an optical element arranged between said first and second reflection optical elements and an optical element arranged between said second reflection optical element and said substrate. That is, if the above-mentioned object plane side optical axis and image plane side optical axis become substantially parallel, there will be an intermediate optical path connecting these optical axes, but in this case the second barrel may be comprised by two barrel components. While not particularly limited to this, more preferably the first reflection optical element is a concave mirror arranged with the reflection surface facing upward and is supported by said frame so that its optical axis becomes substantially parallel with the direction of gravity. If a concave mirror is employed as the first reflection optical element and its optical axis is made substantially the direction of gravity, at least the optical axis of the first optical system will become substantially the direction of gravity and the first barrel will also extend in the direction of gravity. Therefore, the stability of the first barrel on the frame can be improved. Further, if the first barrel and part of the second barrel are made parallel, the part of the second barrel will also extend substantially in the direction of gravity and the stability of not only the first barrel, but also the part of the second barrel on the frame will be improved.
While not particularly limited to this, more preferably said connecting member is fastened to said first barrel and said second barrel on a pair of common tangents connecting the outer periphery of the first barrel and the outer periphery of the second barrel. If the first barrel and the second barrel are connected using the connecting member, outside force from the connecting member will act on the connected portions of the first and second barrels. If an outside force having a normal direction component is applied to a barrel, the barrel will warp, while if an outside force having a tangential direction component is applied to a barrel, that barrel will be twisted. Since the first and second barrels respectively hold the first and second optical systems, warping or twisting of the barrels will lead to warping or twisting of the optical elements, such as the lenses comprising the optical systems. That is, when connecting the first and second barrels, a connecting structure maintaining the true circularity of the barrels and preventing twisting is desirable. By fastening the connecting member to the first barrel and the second barrel on a pair of common tangents connecting the outer periphery of the first barrel and the outer periphery of the second barrel, the forces acting from the connecting member on the first and second barrels will be in tangential directions and not include normal direction components, so the true circularity of the two barrels will be well maintained and warping of the optical elements will be prevented. Further, since the member is fastened on the pair of common tangents, twisting of the first and second barrels will be prevented relatively strongly.
The specific configuration of the connecting member is not particularly limited, but it is possible to employ a connecting member which comprises a rigid plate and where at least one fastening portion among the fastening portions of the rigid plate with the first barrel and the second barrel is made movable in the normal direction. In this case, as a means for making one fastening portion movable in the normal direction, while not particularly limited to this, mention may be made of a means wherein at least one fastening portion of the rigid plate has a hinge movable in the normal direction or a means wherein at least one fastening portion of the rigid plate has a thin portion movable in the normal direction. If the connecting member is comprised by a rigid plate, twisting of the first barrel and the second barrel can be strongly prevented. Considering the manufacturing error of the connecting member and the first and second barrels, however, the fastening portions may not necessarily follow along the direction of the pair of common tangents, so force is liable to act in the normal direction of the barrels. By making a fastening portion of the rigid plate movable in the normal direction, however, even if there is manufacturing error in the rigid plate or the first and second barrels, the fastening portion of the rigid plate can absorb that error and as a result the force acting from the connecting member to the first and second barrels will become only a force in the direction of the common tangents and therefore it will be possible to prevent warping of the barrels due to a force acting in the normal direction of the barrels and in turn prevent warping of the optical elements.
As the specific configuration of the connecting member, in addition to the above rigid plate, it is possible to also employ a connecting member which is comprised of a rigid rod, wherein said rigid rod is fastened to said first barrel and second barrel on a pair of parallel common tangents connecting the outer periphery of said first barrel and the outer periphery of said second barrel, and wherein said first barrel and said second barrel are connected by a second connecting member on another common tangent connecting the outer periphery of the first barrel and the outer periphery of the second barrel. Since the connecting member is a rigid rod, it enables the weight to be reduced and is much easier to manufacture than configuration by a rigid plate. Further, by fastening it on the pair of parallel common tangents connecting the outer peripheries of the first and second barrels, the interval of the first and second barrels in the tangential direction is restrained. On the other hand, in addition, by connecting through use of a second connecting member to another common tangent connecting the outer peripheries of the first and second barrels, it is possible to restrain rotation of the first and second barrels. While not particularly limited to this, the frame and said connecting member are more preferably comprised by materials of substantially equal linear expansion coefficients. If the first and second barrels are integrally supported on the frame, when the frame expands or contracts due to the effects of the temperature, the interval between the first and second barrels at the supporting parts of the frame will change, but by comprising the frame and the connecting member by materials of substantially identical linear expansion coefficients, the connecting part of the connecting member will expand or contract equally as the expansion or contraction at the supporting part of the frame. Accordingly, it will be possible to prevent at least relative displacement of the optical axes between the optical axis of the first barrel and the optical axis of the second barrel. Further, since the frame and the connecting member will expand and contract equally, no stress at all will be applied to the barrels and it will be possible to prevent warping at the optical elements.
While not particularly limited to this, the apparatus is more preferably further provided with a first stage arranged at an object plane side of the projection optical system, a second stage arranged at an image plane side of the projection optical system, and a drive means for driving the first and second stages so as to synchronously move said mask and said substrate by a ratio of speed in accordance with the magnification of the projection optical system. By applying the present invention to a step-and-scan type projection exposure apparatus, it is possible to more effectively manifest the superior imaging characteristics of the present invention.
3. To achieve the second object, according to the present invention, there is provided a projection exposure apparatus for transferring a pattern of a mask on to a substrate, said projection exposure apparatus comprising a projection optical system having an optical element, a frame for supporting a barrel structure holding said projection optical system, a support member for supporting said barrel structure at a position different from the frame, and a partition for isolating the space including at least the outer periphery of the barrel structure from other spaces. In this case, preferably there is further comprising a gas feeder for feeding a high transmittance inert gas (for example, helium gas) to at least one of the insides of said barrel structure and the inside of said partition. Further, while not particularly limited to this, a passage connecting the inside of the barrel structure and the inside of the partition may be formed at a portion of the barrel structure covered by the partition.
According to the present invention, since a partition is provided isolating the space including at least part of the outer periphery of the barrel structure from other spaces, the air-tightness is improved. Accordingly, for example, when sealing in or feeding inert gas to the inside of the barrel structure, there will be less leakage of the inert gas to the outside of the barrel structure and the partition. Due to this, even when using the expensive helium gas as the inert gas, since the amount consumed will become smaller, the operating cost can be lowered. Further, since it is possible to deal with portions of the barrel structure which should be made air-tight altogether to some extent compared with devising measures for air-tightness for each part of said barrel structure where inert gas might leak (for example, the joints of the members comprising the barrel structure or the windows for fine adjustment of the optical characteristics), the air-tightness can be improved without making the configuration of the barrel structure that complicated and the rise in cost can be kept to a minimum. In this case, preferably at least part of the partition is a freely deformable variable partition. By this, even in the case where the partition or barrel structure etc. expands or contracts due to heat at the connecting portions of the partition and barrel structure or frame or other members (hereinafter referred to as the xe2x80x9cbarrel etc.xe2x80x9d), this will be absorbed by the variable partition and the restraining force acting on the barrel structure etc. will become smaller, so it is possible to prevent deterioration of the optical characteristics due to warping of the barrel structure etc. While not particularly limited to this, the variable partition may be made an accordion-like partition comprised of zigzag folds or may be made a sheet-like partition having flexability.
4. To achieve the third object, according to the present invention, there is provided a projection exposure apparatus for transferring a pattern of a mask on to a substrate, including a reflection and refraction projection optical system having optical members including a refraction optical member, a reflection mirror having curvature, and at least two optical path deflection members, said reflection and refraction projection optical system including at least three optical axes and at least three barrels, the at least three barrels each holding at least one optical member among the above optical members, and at least one barrel among the at least three barrels holding two optical path deflection members among the optical path deflection members so that the optical axes before and after bending by the two optical path deflection members become parallel to each other.
As explained above, in the present invention, since optical members having the same optical axes are held altogether by a barrel, each barrel has a single optical axis and therefore adjustment among the plurality of optical axes becomes easy.
5. To achieve the fourth object, according to the present invention, there is provided a projection exposure apparatus for transferring a pattern of a mask on to a substrate, including a reflection and refraction projection optical system having optical members including a refraction optical member, a reflection mirror having curvature, and at least two optical path deflecting members, any two optical path deflecting members among the at least two optical path deflecting members being held by one holding member so that their reflection surfaces become orthogonal to each other.
In this aspect of the present invention, since two optical path deflecting members are held by one holding member so that their reflection surfaces become orthogonal to each other, even if the holding member itself rotates, the relationship between the angle of incidence of the light on these two optical path deflecting members and the angle of emission of the light from the two optical path deflecting members can be maintained and therefore stability is achieved with respect to outside disturbances such as vibration of the projection exposure apparatus itself.